(1) Field of the Invention
The present invention relates to an apparatus and method for detecting an intake air mass flow rate using a Karman vortex air flow sensor for detecting the amount of intake air of an internal combustion engine, more particularly to a method and apparatus for detecting an intake air mass flow rate in an automobile engine provided with an electronically controlled fuel injection apparatus, improving the output signal processing circuit of a Karman vortex air flow sensor of the intake air mass flow rate sensing type.
(2) Description of the Related Art
One of the methods for supplying mixtures of a predetermined air-fuel ratio to the combustion chamber of an automobile engine or other internal combustion engine is to use an electronically controlled fuel injection apparatus of the intake air sensing type. This provides, for example, one or two injectors for injecting fuel into an engine to the throttle body in the case of a single point injection type or a plurality of the same to the intake manifold in the case of a multiple point injection type for control of the opening time of the said injector in accordance with principally the specific volume of intake air of the engine and the engine speed, thereby ensuring that a mixture of the predetermined air-fuel ratio is supplied to the engine combustion chamber.
Air flow sensors used for detection of the specific volume of intake air in an electronically controlled fuel injection apparatus of the intake air sensing type include the flap type, the hot wire type, etc. In recent years, however, the so-called Karman vortex air flow sensor has come into practical application, wherein use is made of the Karman vortex generated alternately near the two sides of a Karman vortex generator provided in an intake passage of an internal combustion engine for the detection of the specific volume of intake air (see Japanese Unexamined Patent Publication (Kokai) No. 57-1913).
Means for using this Karman vortex air flow sensor to accurately detect the volume flow rate of air have been proposed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 58-80523 and Japanese Utility Model Application No. 59-23210.
However, the above-mentioned prior air flow detection means using Karman vortex air flow sensors detect the volume of the air. Therefore, when the engine is operated at a high altitude where the atmospheric pressure is lower than the standard atmospheric pressure (760 mmHg), the mass of intake air actually sucked in decreases by the amount of decrease of the intake air density, leading to deterioration of the air-fuel ratio and, in the worst case, the danger of engine stalling. To prevent this, the detected volume flow rate of the air must be corrected for atmospheric pressure. The prior means for correction of atmospheric pressure of the volume flow rate have been (1) to provide an atmospheric pressure sensor for detecting the atmospheric pressure around the engine and, based on the output of this atmospheric sensor, correcting the volume flow rate of the intake air (Japanese Unexamined Patent Publication (Kokai) No. 57-131841) or (2) feeding back an output signal of, for example, an O.sub.2 sensor provided in the exhaust system for detection of the oxygen concentration in the exhaust gas to effect air-fuel ratio adaptive (or learning) control for controlling the air-fuel ratio and thereby correct the volume flow rate of the intake air (Japanese Unexamined Patent Publication (Kokai) No. 57-26229 and Japanese Unexamined Patent Publication (Kokai) No. 58-48739).
In the prior art of the above-mentioned item (1), however, while the control characteristics of the atmospheric pressure sensor are excellent, the cost is high.
Further, in the prior art of the above-mentioned item (2), while there is no increase in cost, when an engine is operated from a high altitude to a low altitude by engine braking, etc., the supply of fuel is terminated (fuel cut) and the feedback control is suspended. Therefore, when the fuel injection is restarted at the low altitude after a long, continuous fuel cut period, the learning correction coefficient is still held at the value of the high altitude before suspension of feedback control. Consequently, the air-fuel ratio largely deviates to the rich side. As a result, the exhaust gas becomes worse and there is a tendency to engine stalling.